This invention pertains to the field of infrared thermometry and detectors.
Infrared thermometry is a useful means of measuring temperature, in part because it can measure the temperature of an object without making physical contact with the object. Its applications range from measuring the temperature of baths of molten metal to measuring the temperature of the human body by measuring the temperature of the eardrum. In the latter application, temperature measurement accuracies of a fraction of a degree Celsius are routinely achieved.
Infrared thermometry works on the principle that any object emits radiant thermal energy to any other object which is at a different temperature, and the spectral characteristics (essentially, color) of this emitted radiation are indicative of the temperature of the emitting object. The wavelength at which the peak magnitude of the emitted radiation occurs is a function of temperature and the relationship is given by Wien""s Law, which states that the wavelength of peak radiation is inversely proportional to the absolute temperature of the radiating body. This is described in Handbook of Heat Transfer, by Rohsenow and Hartnett (1973) and other reference books. For temperatures around or moderately above room temperature, the wavelength of the radiation peak is in the infrared region of the electromagnetic spectrum. Total radiated energy is given by Planck""s law (or Stefan-Boltzmann law), which includes a proportionality to the fourth power of the absolute temperature.
In radiant heat transfer, the term blackbody receiver or hohlraum is used to refer to a perfect receiver of radiant heat transfer. It is a body to which heat can be radiated, but from which radiant heat is not re-emitted, such as by reflection. The reason for the term black is to denote that ideally the body is fully absorptive, i.e., has an effective absorptivity equal to 1, which is usually associated with the color black. Along with this, the body would have no reflectivity, i.e., reflectivity=0. By definition, for a non-transmissive surface, absorptivity+reflectivity=1. For real materials, absorptivity and reflectivity only approach but never perfectly equal one and zero respectively. This brings on the next strategy for creating a blackbody receiver, which is geometry. A blackbody receiver is typically an enclosure having only a small opening through which thermal radiation enters. For reasons of symmetry and simplicity, the enclosure is most commonly described as a sphere having a small circular opening. Radiation which enters the receiver and undergoes reflection instead of absorption at its first impact with the surface will mostly be reflected to some other portion of the interior surface away from the opening, where most of it will be absorbed except for a still smaller portion which reflects to yet another portion of the interior surface, etc. As a result of those multiple internal reflections at various angles, essentially none of the entering radiation returns to the opening through which it entered. Designing the opening to be small compared to the overall dimensions helps achieve this. As a result, the blackbody receiver operates as an essentially perfect receiver despite the somewhat imperfect absorptivity of the interior surface of the blackbody.
One of the requirements for successful infrared thermometry is to calibrate the measuring device by exposing it to thermal radiation from an object whose temperature is known. This object is referred to as a blackbody source. A blackbody source is essentially a blackbody receiver which is maintained above ambient temperature. All of these considerations just discussed about blackbody receivers apply to a blackbody source, such as a desire for high emissivity (similar to absorptivity) of the surface and also the geometric considerations. A blackbody source typically is the internal surface of a nearly complete hollow sphere with a small hole, and is black inside. Such blackbody sources are commercially available, for example, from Mikron Instruments, Oakland, N.J. and other vendors. In the case of emitting radiation for use in calibration, an important consideration is that the radiating surface of the blackbody source should all be maintained at a uniform temperature. If temperature nonuniformities of the radiating surface were present such a source would still emit thermal radiation, but the spectral peak would be more spread out or less well-defined, and this would make the radiation source less useful for calibration purposes. Similar considerations apply if the radiation source is used for any other form of infrared radiation detector for either calibration or testing.
Present blackbody sources typically use electrical resistive heaters to directly heat the hidden side of the radiating surface of the blackbody source to an elevated temperature. The local temperature distribution of the radiating surface is influenced by possible nonuniformity of heat generation due to heater design and spacing. Locations closer to the heaters will be hotter than locations between the heaters. The local temperature distribution of the radiating surface is also influenced by convective heat transfer to the surrounding gas. The temperature difference between the hot radiating body and the surrounding atmosphere creates flow patterns in the surrounding atmospheric gas and in the hot gas occupying the interior of the blackbody source. These flow patterns are such that, due to buoyancy, hot air spills out of the opening and rises, and its place is taken by cold atmospheric air entering the enclosed concave region.
The entering cool air inside the blackbody source tends to make the lower part of the radiating surface cooler than the upper part. These effects can be expected to become more prominent as the size of the blackbody source increases. There are also two possible effects tending to even out or smooth out the temperature distribution. One of these is radiant heat transfer between different portions of the radiating surface facing each other. The effect rapidly increases in importance with temperature (as the fourth power of the absolute temperature) and is described by the view factor which is tabulated as a function of geometry in reference books such as Serafim and Hottel. Another process tending to even out temperature nonuniformities is conduction of heat laterally along the surface of the blackbody radiating surface.
All of these considerations affect the uniformity of the temperature of the entire interior surface of the blackbody source, and the first two impose limitations on the achievable uniformity. Accordingly, it would be useful to have a blackbody source having improved temperature uniformity of the radiating surface.